Thymus

The thymus is a specialized primary lymphoid organ within the immune system. Its main purpose is to facilitate the maturation of T cells. These cells are vital to the adaptive immune system. This system allows the body to adapt to specific foreign invaders. The thymus is located in the anterior superior mediastinum. This is the upper front part of the chest. It sits behind the sternum and in front of the heart.

The organ is composed of two lobes that meet at the upper midline. Each lobe is covered by a protective capsule. Inside, the thymus is divided into two distinct regions. The outer region is called the cortex. The inner region is known as the medulla. These regions contain a network of epithelial cells. These cells support immature T cells, which are called thymocytes. Other cells like macrophages and dendritic cells are also present.

Thymocyte maturation is a precise, step-by-step process. It begins when hematopoietic precursors migrate from the bone marrow into the thymus. Once inside, these cells undergo V(D)J recombination. This genetic process creates unique T cell receptors. The cells then move into the cortex for positive selection. In this stage, thymocytes must interact with MHC immune receptors on epithelial cells. If a cell cannot react to the MHC, it undergoes apoptosis, or programmed cell death.

After passing positive selection, cells move to the medulla for negative selection. This step is crucial for preventing autoimmune diseases. In the medulla, epithelial cells and dendritic cells express proteins from throughout the body. A gene called AIRE helps stimulate this expression. Thymocytes that react too strongly to these "self" antigens are eliminated through apoptosis. This ensures that mature T cells do not attack the body's own tissues. Some cells that survive this stage become T regulatory cells.

As T cells mature, they express either CD4 or CD8 proteins, but not both. A cell that binds mostly to MHC class I becomes a cytotoxic CD8 positive T cell. A cell that binds mostly to MHC class II becomes a CD4 positive T cell. Once fully mature, these cells emigrate from the thymus. They enter the peripheral circulation to provide cell-mediated immunity. This process is aided by hormones and cytokines like thymulin and thymosins.

The thymus changes significantly throughout a person's life. At birth, it is about 4 to 6 cm long and 1 cm thick. It grows until puberty, reaching a mass of approximately 40 to 50 grams. After puberty, the organ undergoes thymic involution. This is a process where the thymus decreases in size and activity. Fatty and connective tissues gradually replace the functional thymic tissue. By old age, the thymus may weigh only 5 to 15 grams.

This process of involution is linked to rising levels of sex hormones. It is an evolutionary process seen in most vertebrates with a thymus. While the thymus shrinks, some T cell development continues into adulthood. However, abnormalities can cause serious health issues. Problems with thymus development can lead to severe immunodeficiency. For example, DiGeorge syndrome involves a failure of the third and fourth pharyngeal pouches to develop. This can result in a lack of a functional thymus.

Historically, the thymus has been known since the time of the Ancient Greeks. However, its specific role in the immune system was not clear until the 1960s. Medical issues can also arise from growths in the tissue. A thymoma is a type of cancer of the thymus tissue. Lymphoma is a cancer arising from immature lymphocytes. Doctors may sometimes perform a thymectomy, which is the surgical removal of the organ. Understanding the thymus helps us understand how our bodies defend themselves.